The present invention generally relates to switching power supply controllers and more particularly, to electronics for switching power supplies.
In general, a switching regulator includes a switching circuit turned on and off at a high frequency, and the ratio of the on to off periods (i.e., the duty ratio) is controlled to regulate the output voltage or current.
Switching power supplies are often operated at a fixed switching frequency to gain the advantages of predictable Electromagnetic Interference (EMI) performance. Fixed switching frequency power supplies employing peak current-mode output voltage control or voltage-mode controlled supplies employing pulse-by-pulse output current limiting are susceptible to loss of output voltage or current control (i.e., loss of regulation) under certain abnormal conditions such as an abnormally high input voltage, a shorted output, and an abnormally light load (the last of which is applicable only for non-synchronously rectified supplies). Degradation of power converter control over the output voltage can cause malfunction to, or damage of, the output load—causing functional system failure and possibly a safety hazard. While it is important to maintain power converter control during such abnormal conditions, the power switch turn-on pulsewidths required to do so may be shorter than the minimum pulsewidth that the control circuit can produce.